This study is the first to directly compare native and periprosthetic distal femur fractures. PDFF were found to be commonly isolated injuries with complete metaphyseal comminution, affecting elderly women and those with low bone quality. NDFF, on the other hand, tended to occur in younger patients with less metaphyseal comminution and additional fractures. NDFF had increased revision reoperation rates compared to PDFF, specifically for I&D of the femur fracture of interest. In addition to having generally more open fractures in the NDFF group, patients with open fractures within the NDFF group were more likely to result in infection requiring I&D than those with closed fractures. Multivariate regression analysis revealed that NDFF were an independent risk factor for reoperation, specifically I&D of the fracture of interest, compared to PDFF.

The present study reports a PDFF gender distribution similar to Elsoe et al., who documented a sample of mostly females with a mean age of 77 years old [8]. Generally, our PDFF sample was characterized by a high prevalence of low bone density. Additionally, the PDFF cohort had a higher rate of low bone density compared to the NDFF cohort, which can be explained by the higher average age and the female majority in the PDFF group [20]. Low bone density has been highlighted as a risk factor for femur fractures in past studies, and low-energy distal femur fractures are now considered fragility fractures [6, 21]. Although trauma mechanisms were not formally investigated in our study, we observed that PDFF were mostly isolated injuries, which is more suggestive of a low-energy trauma mechanism as proposed by prior studies [7, 22].

The most common fracture pattern for PDFF was extraarticular with complete metaphyseal comminution (AO/OTA 33 A.3). The increased metaphyseal comminution is likely related to both the presence of low bone quality and the TKA implant affecting the stress concentration locations of the fracture. Low bone quality leads to an overall decreased tolerance for withstanding forces. Additionally, with the presence of a TKA, the fracture cannot propagate into the joint, and more energy may be imparted to the metaphysis.

While previous studies have shown that rIMN and lateral locked plating are the most common methods for treating PDFF, our study revealed that rIMN and NPC fixation are the most common methods utilized at our institution [10, 23, 24]. Further analysis of NPC fixation rates revealed an increase in prevalence from 3.3% in 2015 to 13.3% of all fixation constructs used for PDFF in 2019. There was no significant difference in malunion/nonunion rates for those treated with NPC fixation compared to those treated with rIMN or plate fixation alone for either NDFF or PDFF. Regarding postoperative weightbearing status, there was no significant difference in WBAT assignment for NDFF NPC fixation compared to nail or plate fixation (p = 0.88). This was likely confounded by the high prevalence of polytrauma and concomitant fractures in this group, which would have limited weightbearing. However, PDFF treated with NPC fixation were significantly more likely to be WBAT compared to those treated with nail or plate fixation alone (p < 0.05). NPC fixation is a recently being used as an ideal treatment for osteoporotic distal femur fractures (both PDFF and NDFF) due to the balanced energy distribution between bone and implant [25]. Currently, only small cohort studies exist which have found no significant difference in nonunion rates between NPC compared to nail or plate fixation, although a multicenter propensity analysis suggested there may be significantly lower nonunion rates in DFF treated with NPC fixation [25,26,27]. In addition to potentially reducing the risk of nonunion, many surgeons see a biomechanical advantage of combination fixation to facilitate early weightbearing in elderly patients [25]. It is of the authors’ opinion that the results reflect an increasing preference for this treatment by orthopedic surgeons at our institution to stabilize fractures in elderly patients with low bone density to facilitate earlier mobilization/weight bearing.

We have identified a large NDFF population of middle-aged patients (average age = 57 years old) with a balanced sex distribution. The most common fracture pattern consisted of metaphyseal comminution with intra-articular extension (AO/OTA 33 C.2), suggesting a predominantly high-energy trauma mechanism. This contrasts with the findings of Roy et al., who reported a small sample of NDFF at a level-1 trauma center (n = 87) consisting mostly of middle-aged female patients with comparable rates of high-energy (47%) and low-energy injuries (53%) [28]. These differences may be reflective of differences in the demographics of the catchment area that our institution serves.

In contrast to prior literature, which reports coronal plane (AO/OTA 33B.3) fractures representing 38% of all partial articular (AO/OTA 33B) native fractures, our study reports an overall rate of 14% for 33B fractures with a majority being fractures of the lateral condyle (AO/OTA 33B.1) [29]. The mechanism of coronal plane fractures consists of vertical shear forces experienced during high-energy trauma, which is consistent with our findings in the NDFF population. The difference in our reported prevalence of 33B.3 fractures is less likely to be explained by low detection as CT scans were obtained for all patients. These demographic and injury severity differences may be reflective of population differences in sampling; however, we report an NDFF cohort that is much larger than the previously mentioned study, with greater potential for generalizability.

Regarding the treatment of NDFF, rIMN was also the most common fixation method used, followed by lateral plating, which contrasts with previous studies which report plating as the most common fixation for NDFF [7, 11, 12]. Additionally, NPC was the third most common construct employed, which may reflect its increasing popularity as a treatment alternative for distal femur fractures as well as institutional preference for nailing.

Quality of reduction was improved in the PDFF cohort compared to the NDFF cohort, based on normative values of alignment. This may be due to the simplicity of the fractures as PDFF were all type A fractures whereas NDFF commonly had intra-articular components. Additionally, the TKA implants force a certain nail start point given the box component with less variation so perhaps the nail is more on axis. However, nailing of these fractures has been previously associated with malalignment [30, 31]. Finally, the increase in NPC versus lateral plating alone may account for some of the differences as the tendency for malreduction with a single lateral locked plate is well documented [32]. We did not observe any difference in nonunion, similar to prior studies but with an overall lower rate [30].

Contrary to our hypothesis, there were notable differences in outcomes between NDFF and PDFF. NDFF had significantly longer length of stays and were more likely to return to the operating room for additional treatment of the femur fracture compared to PDFF. The most common fracture pattern seen in NDFF undergoing reoperation was complete articular (AO/OTA 33 C), and the most common etiology for UROR was for irrigation and debridement of an initially open fracture due to infection risk, which is reflective of the more severe soft tissue injury and propensity for open fractures. Upfill-Brown et al. previously conducted a large retrospective review which found no significant differences in 30-day reoperation rates between PDFF and NDFF [13]. However, their study did not account for mechanism or fracture complexity. The differences in length of stay and UROR rates between the PDFF and NDFF groups noted in our study can be explained by the high prevalence of polytrauma, additional fractures, and increased fracture complexity (AO/OTA 33 C) in the NDFF group. Additionally, Kaufman et al. studied outcomes in a matched cohort of PDFF and NDFF and found that when controlling for age, sex, and injury severity, there was no significant difference in length of stay or < 90-day readmission rates between the two injuries [14]. The results of Kaufman et al. and our study support the notion that the risk for readmission is more closely tied to population-specific risk factors such as demographics, mechanism, and additional injuries than to the presence of periprosthetic fractures.

There are several notable strengths to this study. This investigation encompasses recent patient data from a large population spanning 8 years followed longitudinally. Our study takes into consideration the quality of reduction when assessing outcomes for PDFF and NDFF. Limitations to the study include retrospective-single site sampling and an unmatched patient cohort. Reduction quality measurements were made by a single grader, potentially introducing information bias. Our results reflect the treatment of distal femur fractures at a level-1 trauma center, and it is unclear whether similar results would be observed at a community hospital or arthroplasty surgical center. Finally, the addition of patient-reported outcomes would be beneficial but were not collected during the time frame investigated.